ADL Barrier Idiom

adl_barrier_idiom.cpp

// What you do when you want to prevent ADL from kicking in, and force your
// users to use fully qualified names.

namespace module
{
struct Foo { int a, b; }; // struct defined in one namespace

namespace functions
{
void Flaggerbify(Foo* foo); // function defined in another
}

using namespace module::functions; // we pretend functions are in same namespace (the public one)
} // namespace module

// Usage code:
int main()
{
  module::Foo a_foo;
#if ADL_NOT_PREVENTED
  // This call would work if Faggerbify was really in the same namespace as module::Foo, due to ADL
  // @url: http://en.cppreference.com/w/cpp/language/adl
  Flaggerbify(&a_foo);
#endif
  // Instead, the user has to do this (or import the whole module)
  module::Flaggerbify(&a_foo);
};

// Implementation, not interesting:
namespace module
{
namespace functions
{
void Flaggerbify(Foo* foo)
{
  foo->a *= -1;
  foo->b *= -1;
}
}
}